Although sheer "number crunching" is an important application of digital electronics, the real power of digital techniques is seen when digital methods are applied to analog (or linear) signals and processes.
The ability to efficiently and effectively combine and convert signals among several different formats enables the combining of analog and digital techniques to provide powerful solutions to many challenging problems, the significance of which can most clearly be seen when bringing digital signals on and off circuit boards, in and out of instruments, and through cables.
It is often necessary to convert an analog signal to an accurate digital number proportional to its amplitude, and vice versa. Such a conversion is essential in any application in which a computer or other processor driven system is responsible for logging or controlling experimental processes, or where digital techniques are used to perform "normal" analog tasks.
An example of a "normal" analog task would be an application in which analog information is converted to an intermediate digital format for error and noise free transmission, such as "digital audio" or "pulse code modulation" (PCM) applications.
A second example would be the establishment of an output power source (such as a voltage or current level) as a function of an input signal. This is especially important where a video signal is controlled by an analog or digital input voltage level.
A wide variety of measurement instruments, including ordinary bench instruments, such as digital multimeters, and more exotic instruments, such as transient averagers, "glitch catchers," and digital memory oscilloscopes, as well as signal-generation and processing instruments, such as digital waveform synthesizers and data encryption devices, also make significant use of analog to digital and digital to analog conversion.
Two popular methods for converting digital to analog signals exist. The first method uses scaled, or binary weighted, resistors coupled with a summing junction, while the second uses an R-2R ladder (a resistor and signal source network which generates binary scaled signals). In either case, the circuit accepts an n-bit digital word and produces an analog signal. Neither is the preferred method, and the decision as to which method to implement most often turns upon the application involved.
The advantage of the scaled resistor technique is its speed. Its drawbacks, however, are (1) that the conversion is not precise; and (2) in circumstances when more than a few bits of information are to be converted, the implementation becomes awkward and cumbersome. For example, in a 12-bit converter circuit, a range of resistor values of 2000:1 is required.
The R-2R ladder technique, on the other hand, provides an elegant solution in that only two resistor values are required regardless of the number of bits of information to be converted. Thus, the advantages of the R-2R ladder are that (1) the implementation is simplified by limiting the range of resistor values to two; and (2) the implementation is considered precise. The disadvantage of the R-2R ladder technique is that this implementation is considered slow.
In the area of analog to digital conversion, there are approximately a half dozen techniques for converting analog signals to an equivalent digital format. As was the case with digital to analog converters, each technique has its own peculiar advantages and limitations. Similarly, as with digital to analog conversion, there exists an implementation decision which turns largely upon a trade-off between speed and precision.
In addition, all of the previously discussed conversion techniques are incapable of guaranteeing full monotonicity as a particular conversion technique processes sequential signals. Full monotonic conversion is the conversion of input signals such that as adjacent input signals change in value, output signal "glitches" are nonexistent. Such a "glitch" would be present, for example, in a presently available output voltage control system where the output voltage does not always transition between adjacent signals in the same direction as the change in value of the input signal. For example, a "glitch" may occur in an output signal emanating from a converter when an increase from one input signal to the subsequent input signal results in the output signal first decreasing to a reference level in-between the adjacent input signals before the output signal increases to correspond to the new increased input signal. Furthermore, "glitches" may cause a "flash" to occur on a video monitor, a "flash" being the surge seen on a video monitor displaying an analog signal non-monotonically converted from a digital base.
Thus, the need for such a monotonic conversion technique is an essential ingredient for the generation of analog displays by digital instrument, e.g., a meter indication or XY display (or plot) created by a computer, or more particularly in real-time video processing.
Therefore, there exists a need in the art for a system and method for converting signals received in a first format to a second format which is both efficient and highly accurate, e.g., fast and precise.
There exists a further need in the art for a system and method for converting subsequently received signals such that changes in adjacent signals' values are monotonic, thus eliminating both "flashes" and "glitches."
There exists a still further need in the art for a system and method for converting sequentially received signals which may be compactly, or densely, implemented within an integrated circuit structure.
There is a still further need in the art for a means by which to convert signals received in a first format to a second format which is both accurate and fast, and is not restrained by the size of the signal received in the first format.